Recent, large-scale changes in the extratropical southern hemisphere atmospheric circulation

The zonal anomalies in the 500-hPa geopotential height field of the Southern Hemisphere are examined using the National Centers for Environmental Prediction - National Center for Atmospheric Research reanalyses, 1958 - 96. The zonal anomalies were constructed by removing the zonal mean from each grid point, and principal components analysis was performed on the resulting spatial anomalies. The principal components of the leading EOF of the seasons March - April - May (MAM) to September - October - November ( SON) indicate that a change has occurred in the extratropical, large-scale, atmospheric circulation of the Southern Hemisphere. It is estimated to have begun after 1975 and is limited to the southern late fall, winter, and early spring. The elements of this change are a deeper Pacific trough and higher Indian Ocean ridge at subtropical latitudes. At higher latitudes the ridge over the Pacific strengthened and shifted east while troughing occurred in the southwest Atlantic Weddell Sea region. Consistent with the changes in the zonal anomalies, zonal wave 1 experienced a large increase in amplitude in the late winter - early spring. On the other hand, zonal wave 3 reached its maximum one month earlier, shrank more quickly, and reached lower-than-average amplitudes in spring after 1975.